Estimation of suspended sediment concentration and yield using linear models,random forests and quantile regression forests

For sediment yield estimation, intermittent measurements of suspended sediment concentration (SSC) have to be interpolated to derive a continuous sedigraph. Traditionally, sediment rating curves (SRCs) based on univariate linear regression of discharge and SSC (or the logarithms thereof) are used but alternative approaches (e.g. fuzzy logic, artificial neural networks, etc.) exist. This paper presents a comparison of the applicability of traditional SRCs, generalized linear models (GLMs) and non‐parametric regression using Random Forests (RF) and Quantile Regression Forests (QRF) applied to a dataset of SSC obtained for four subcatchments (0·08, 41, 145 and 445 km²) in the Central Spanish Pyrenees. The observed SSCs are highly variable and range over six orders of magnitude. For these data, traditional SRCs performed inadequately due to the over‐simplification of relating SSC solely to discharge. Instead, the multitude of acting processes required more flexibility to model these nonlinear relationships. Thus, alternative advanced machine learning techniques that have been successfully applied in other disciplines were tested. GLMs provide the option of including other relevant process variables (e.g. rainfall intensities and temporal information) but require the selection of the most appropriate predictors. For the given datasets, the investigated variable selection methods produced inconsistent results. All proposed GLMs showed an inferior performance, whereas RF and QRF proved to be very robust and performed favourably for reproducing sediment dynamics. QRF additionally provides estimates on the accuracy of the predictions and thus allows the assessment of uncertainties in the estimated sediment yield that is not commonly found in other methods. The capabilities of RF and QRF concerning the interpretation of predictor effects are also outlined. Copyright © 2008 John Wiley & Sons, Ltd.     

Noble gas composition,cosmic-ray exposure age, 39Ar-40Ar,and I-Xe analyses of ungrouped achondrite NWA 7325

Northwest Africa (NWA) 7325 is an anomalous achondrite that experienced episodes of large-degree melt extraction and interaction with melt under reducing conditions. Its composition led to speculations about a Mercurian origin and provoked a series of studies of this meteorite. We present the noble gas composition, and results of ⁴⁰Ar/³⁹Ar and ¹²⁹I-¹²⁹Xe studies of whole rock splits of NWA 7325. The light noble gases are dominated by cosmogenic isotopes. ²¹Ne and ³⁸Ar cosmic-ray exposure ages are 25.6 and 18.9 Ma, respectively, when calculated with a nominal whole rock composition. This ³⁸Ar age is in reasonable agreement with a cosmic-ray exposure age of 17.5 Ma derived in our ⁴⁰Ar/³⁹Ar dating study. Due to the low K-content of 19 ± 1 ppm and high Ca-content of approximately 12.40 ± 0.15 wt%, no reliable ⁴⁰Ar/³⁹Ar age could be determined. The integrated age strongly depends on the choice of an initial ⁴⁰Ar/³⁶Ar ratio. An air-like component is dominant in lower temperature extractions and assuming air ⁴⁰Ar/³⁶Ar for the trapped component results in a calculated integrated age of 3200 ± 260 (1σ) Ma. This may represent the upper age limit for a major reheating event affecting the K-Ar system. Results of ¹²⁹I-¹²⁹Xe dating give no useful chronological information, i.e., no isochron is observed. Considering the highest ¹²⁹Xe*/¹²⁸Xe_I ratio as equivalent to a lower age limit, we calculate an I-Xe age of about 4536 Ma. In addition, elevated ¹²⁹Xe/¹³²Xe ratios of up to 1.65 ± 0.18 in higher temperature extractions indicate an early formation of NWA 7325, with subsequent disturbance of the I-Xe system.     

Biotic controls on shallow translational landslides

In undisturbed tropical montane rainforests massive organic layers accommodate the majority of roots and only a small fraction of roots penetrate the mineral soil. We investigated the contribution of vegetation to slope stability in such environments by modifying a standard model for slope stability to include an organic layer with distinct mechanical properties. The importance of individual model parameters was evaluated using detailed measurements of soil and vegetation properties to reproduce the observed depth of 11 shallow landslides in the Andes of southern Ecuador. By distinguishing mineral soil, organic layer and above‐ground biomass, it is shown that in this environment vegetation provides a destabilizing effect mainly due to its contribution to the mass of the organic layer (up to 973 t ha^{? 1} under wet conditions). Sensitivity analysis shows that the destabilizing effect of the mass of soil and vegetation can only be effective on slopes steeper than 37.9°. This situation applies to 36% of the study area. Thus, on the steep slopes of this megadiverse ecosystem, the mass of the growing forest promotes landsliding, which in turn promotes a new cycle of succession. This feedback mechanism is worth consideration in further investigations of the impact of landslides on plant diversity in similar environments. Copyright © 2012 John Wiley & Sons, Ltd.     

A spatially explicit scenario-driven model of adaptive capacity to global change in Europe

Traditional impact models combine exposure in the form of scenarios and sensitivity in the form of parameters, providing potential impacts of global change as model outputs. However, adaptive capacity is rarely addressed in these models. This paper presents the first spatially explicit scenario-driven model of adaptive capacity, which can be combined with impact models to support quantitative vulnerability assessment. The adaptive capacity model is based on twelve socio-economic indicators, each of which is projected into the future using four global environmental change scenarios, and then aggregated into an adaptive capacity index in a stepwise approach using fuzzy set theory. The adaptive capacity model provides insight into broad patterns of adaptive capacity across Europe, the relative importance of the various determinants of adaptive capacity, and how adaptive capacity changes over time under different social and economic assumptions. As such it provides a context for the implementation of specific adaptation measures. This could improve integrated assessment models and could be extended to other regions. However, there is a clear need for a better theoretical understanding of the adaptive capacity concept, and its relationship to the actual implementation of adaptation measures. This requires more empirical research and coordinated meta-analyses across regions and economic sectors, and the development of bottom-up modelling techniques that can incorporate human decision making.     

Effect of injected CO2 on geochemical alteration of the Altmark gas reservoir in Germany

Capturing CO₂ from point sources and storing it in geologic formations is a potential option for allaying the CO₂ level in the atmosphere. In order to evaluate the effect of geological storage of CO₂ on rock-water interaction, batch experiments were performed on sandstone samples taken from the Altmark reservoir, Germany, under in situ conditions of 125 °C and 50 bar CO₂ partial pressure. Two sets of experiments were performed on pulverized sample material placed inside a closed batch reactor in (a) CO₂ saturated and (b) CO₂ free environment for 5, 9 and 14 days. A 3M NaCl brine was used in both cases to mimic the reservoir formation water. For the “CO₂ free” environment, Ar was used as a pressure medium. The sandstone was mainly composed of quartz, feldspars, anhydrite, calcite, illite and chlorite minerals. Chemical analyses of the liquid phase suggested dissolution of both calcite and anhydrite in both cases. However, dissolution of calcite was more pronounced in the presence of CO₂. In addition, the presence of CO₂ enhanced dissolution of feldspar minerals. Solid phase analysis by X-ray diffraction and Mössbauer spectroscopy did not show any secondary mineral precipitation. Moreover, Mössbauer analysis did not show any evidence of significant changes in redox conditions. Calculations of total dissolved solids’ concentrations indicated that the extent of mineral dissolution was enhanced by a factor of approximately 1.5 during the injection of CO₂, which might improve the injectivity and storage capacity of the targeted reservoir. The experimental data provide a basis for numerical simulations to evaluate the effect of injected CO₂ on long-term geochemical alteration at reservoir scale.     

Polarimetry of solar pores

We address the magnetic field structure of solar pores. The data were obtained at the Gregory Coudè telescope at Izaña using the AT1 CCD camera system to observe pores with three spectral lines: one magnetically sensitive line, recording all 4 Stokes profiles, and two g = 0 lines where only the intensity profiles were measured. The data reduction included the standard procedure (removing dark current and flatfielding) as well as destretching of the polarimetric spectra and removing the non-magnetic straylight by means of a 2-d deconvolution of the observed intensity variation using a Lucy-Richardson restoration algorithm. In the following analysis we first determined the temperature- and pressure stratification of the pore using the g = 0 lines and then applied an inversion of the Stokes profiles to get the parameters of the magnetic field. Across the pore we find a strong variation of the resulting field strength as well as of the inclination and the azimuth, consistent with the assumption of a canopy forming in the higher atmosphere.     

The influence of capacitor effects on the surface potential of satellites with partially insulating surfaces in the solar wind

A simple model is presented for the estimation of the surface potential of satellites with insulating coating. Any small area F of the surface is represented by a capacitor, which is charged up by the plasma and photoelectron currents. Using simple approximations for the currents, the surface potential is calculated for plasma parameters supposed to describe the solar wind between 0.3 a.u. and 1 a.u. distance from the Sun.The potential of the shaded side of the satellite is found to depend strongly on the capacitance of the surface. High capacitances or high spin frequency for the spacecraft can be used to limit the potential differences on the surface. In any case the angular surface potential distribution is asymmetric with respect to the solar wind velocity.     

Nachocho — an example of the change of the living sphere of the Maasai of Northern Tanzania

The example of Nachocho's enkang shows the recent socio-economic changes taking place in the southern Maasai Steppe of N Tanzania. The initial point is the traditional living community in the enkang and the form of pastoralism which are the basis of the economic and social security systems of the Maasai. The state land use policy, however, leads to the strong restriction of the grazing potential. In order to secure land titles for themselves, the Maasai try to pursue agropastoral activities, although they have had, until recently, no contact or experience with farming at all. In this transitional situation, their old pastoral economic basis is strongly endangered, although no useful alternative has been found, so far. For the Maasai of the Tanzanian Steppe a process of impoverishment is thus being triggered off.